Apparatus and method for wet screening



March 28, 1961 F. J. FONTEI'N 2,976,996

APPARATUS AND METHOD FOR WET SCREENING Filed June 18, 1956 1 PatentedMar. 28, 1961 APPARATUS AND METHOD FOR WET SCREENING Freerk .l. Fontein,Heerlen, Netherlands, assignor to Stamicarbon N.V., Heerlen, NetherlandsFiled June 18, 1956, Ser. No. 591,929 Claims priority, applicationNetherlands June 17, 1955 2 Claims. (Cl. 209-355) This invention relatesto particle separation and more particularly to an apparatus and methodfor wet screening suspended mixtures of fine particles differing ingrain size. This application constitutes a continuation-in-part of mycopending application Serial No. 475,251, filed December 14, 1954, andissued December 8, 1959, as Patent No. 2,916,142.

In the above mentioned patent, there is disclosed an apparatus forscreening suspensions of mixtures of particles differing in grain sizewhich is termed a sieve bend. 'Ihe sieve bend comprises a non-continuouscurved separating body or'screening deck having a feed device openingtangentially thereto so as to supply the suspended mixture perpendicularto the generatrix of the screening deck. The feed is connected at oneend of the deck and a collecting device for the fraction retained by thescreen is connected to the other end. The size of the apertures or slotsin the screening deck, measured in the direction of flow of the materialto be treated, is equal to or smaller than the size of the same barsmeasured in the direction normal to the direction of flow. The materialto be screened is supplied at the feed side at a rate sufilciently highto effect a fair degree of screening or dewatering, whichever may berequired, and to ensure the removal of the coarse particles at thedelivery end.

A distinction according to shape is made between the curved sieve bendand the flat sieve bend. The radius of curvature of the latter isinfinite. In a preferred embodiment, the screening deck is formed by abar screen having its bars substantially perpendicular to the directionof flow of the material to be screened. In the curved sieve bend thebars, which run perpendicular to the generatrices of the curvedscreening deck, are straight. The flat sieve bend is preferably mountedat an angle of 30 to 60 to the horizontal plane. One of the advantagesof the sieve bend is that it effectively resists blockage. The diameterof the largest particle that is allowed to pass through the screen willgenerally not be greater than one-half of the width of the slots betweenthe bars. Only in some cases, especially when screening at very finesizes, does blockage occur.

I have found that by leading the underflow of a sieve bend over afollowing sieve bend with a smaller width of slot than that of thefirst, it is possible to reclassify the finer particles in a simple andconvenient manner.

Accordingly, it is an object of the present invention to provide aprocess of the type described in which separation of very fine sizedparticles can be obtained without blockage.

Another object of the present invention is the provision of a processfor wet screening suspended mixtures of very fine sized particles inwhich the mixture is separated on a first sieve bend and the undersizereceived 2 therefrom is subsequently separated on a second sieve bendconnected in series with the first, the latter having a slot width atmost 1.9 times the slot width of the second sieve bend.

A further object of the present invention is the provision of a processof the type described in which successive wet screening operations areperformed without the necessity of using additional liquid.

A still further object of the present invention is the provision of aprocess of the type described in which the undersize particles obtainedin an initial wet screening operation are subsequently separated on asieve bend having screen apertures larger than the maximum size of theparticles supplied thereto so as to exclude the possibility of blockage.

A still further object of the present invention is the provision of anapparatus for separating suspended mixtures of particles of very smallsize which includes a pair of sieve bends connected in series so thatthe undersize issuing from one is fed to the other and in which therelative slot widths of the sieve bends are so related as to exclude thepossibility of blockage on the following sieve bend or sieve bends.

These and other objects of the present invention will become moreapparent during the course of the following detailed description andappended claims.

The invention may best be understood with reference to the accompanyingdrawing in which a schematic view of one embodiment of an apparatusconstructed in accordance with the principles of the present inventionis shown.

The present invention relates also to an apparatus for realizing thesaid process, in which a number of sieve bends are arranged in, seriesand the undersize of any preceding sieve bend is again subjected toscreening on a following sieve bend. According to the invention, thewidth of slot of any preceding sieve bend is at most 1.9 times the widthof slot of the following screen. By way of example the invention isexplained with reference to the drawing, which is a diagram of anarrangement of a number of series-connected sieve bends.

As shown in the drawing, the apparatus consists of threeseries-connected sieve bends, generally indicated 1, 2 and 3.Preferably, these sieve bends are constructed in accordance withteachings of the aforesaid Patent No. 2,916,142, although it will beunderstood that the invention also contemplates the utilization of flatscreening decks placed at an angle to the horizontal plane. Thus, sievebend 1 includes a cylindrically curved separating body or screening deck5 which may be formed by bars or the like spaced apart so as to provideseparating or screening apertures in the manner indicated in Patent No.2,916,142. The raw material, as for example, a suspension of coal inwater, is tangentially supplied, as at 4, to the concave side thescreening deck 5 at a predetermined minimum initial velocity. Thegreater part of the liquid, together with that part of the solidparticles, having a grain size not larger than one-half of the width ofslot, drops into a collecting tank 6 and is discharged at 7 onto thesecond sieve bend 2. The overflow from the sieve bend 1 is discharged at8. The underflow of screen 1, which is discharged at 7, is tangentiallysupplied onto a similarly constructed curved screening deck 9 of thesieve bend 2, which has a width of screening aperture or slot of atleast 1/ 1.9 times that of the slot width of the sieve bend-1. It canthus be seen that the slots in the screen deck 9 are larger than thesize of the largest particles contained in the underflow of screen 1.The underflow of sieve bend 2 is, in its turn, discharged at 10 ontoanother similarly constructed screening deck 11 of sieve bend 3. Theoverflow from the screening deck 9 is discharged at 12. The same processis repeated on sieve bend 3 where the underflow is discharged at 13 andthe overflow at 14. The slot width of screening deck 11 bears the samerelation to the slot width of screening deck 9 as the latter does to theslot width of the screening deck 5.

Example At 4 fine coal with particle size of G8 mm., suspended in water,was supplied. The screening deck had slots of 1.5 mm, the screening deck9 had slots of 1 mm. and the screening deck 11 had slots of 0.6 mm.

Sieve bend it operated at a particle size of about 1.5+2 =0.75 mm.Consequentl the underflow was smaller than the width of slot ofscreening deck 9, which was 1 mm. This sieve bend made a cut at aparticle size of about 0.5 mm., which size was smaller than the width ofslot of screening deck 11, this being 0.6 mm.

It can thus be seen that there has been provided an apparatus and methodfor separating suspended mixtures of solid particles of very small sizewhich embodies the utilization of two or more series connected sievebends in which the feed of the subsequent sieve bends is formed by theunderfiow of the succeeding sieve bend. The slot width of the succeedingsieve bend being at most 1.9 times the slot width of the following sievebend. Preferred ratios between these slot widths are between 1.4 and1.5. It will be understood that the term slot width as herein useddenotes the dimension of an opening measured in the direction of fiow ofthe material to be treated.

The arrangement outlined above secures definite advantages. Since theunderfiovv of the sieve bend contains the greater part of the liquid, inwhich the solid particles are suspended, a good operation of each of thefollowing sieve bends is ensured and it is not necessary to use anadditional amount of liquid. Moreover, due to the abovementioned ratiobetween the widths of slot, any of the following sieve bends will besupplied with particles of sizes smaller than its width of slot, so thatblockage is ex cluded.

It thus will be seen that the objects of this invention have been fullyand effectively accomplished. It will be realized, however, that theforegoing specific embodiment has been shown and described only for thepurpose of illustrating the principles of this invention and is subjectto extensive change without departure from such principles. Therefore,this invention includes all modifications encompassed within the spiritand scope of the following claims.

I claim:

1. An apparatus for separating particles according to size comprisingfirst and second separating bodies each having a feed end, a dischargeend, a separating surface generally conforming to a surface generated bymoving a line parallel to itself so that a given point on the line movesin a plane perpendicular to the moving line, and

surfaces spaced in the direction of movement of said line defining aplurality of separating apertures in said separating surface, the widthof the separating apertures of said first separating body measured inthe direction of movement of said line being atmost 1.9 times the widthof the apertures of said second separating body, means for feeding solidparticles and a suspending liquid in layer formation to the feed end ofsaid first separating body at a predetermined minimum initial velocityin the direction of the separating surface thereof substantiallyperpendicular to the line generating the same so that the aperturedefining surfaces of said first separating body facing in the directionof the feed end will block successive layers of a thicknesssubstantially one-fourth the width of the associated separatingapertures and particles in the block layers of a size less than twicethe thickness thereof and hence less than one-half the width of theassociated separating apertures will pass therethrough and the remainingparticles will pass onto the discharge end of said first separatingbody, and means for collecting an undersize fraction passing through theapertures of said first separating body and feeding the same in layerformation to the feed end of said second separating body at apredetermined minimum initial velocity in the direction of theseparating surface thereof substantially perpendicular to the linegenerating the same so that the aperture defining surfaces of saidsecond separating body facing in the direction of the feed end willblock successive layers of a thickness substantially less than the widthof the associated apertures and particles in said block layers of a sizeless than twice the thickness thereof and substantially less than thewidth of the associated separating apertures will pass therethrough andthe remaining particles will pass on to the discharge end of said secondseparating body.

2. A process of separating a mixture of particles according to sizewhich comprises the steps of establishing at a firstreceiving position alayer formation fiow of particles and a suspending liquid having aminimum initial velocity; supporting one surface thereof to locallydefine the direction of travel of said layer formation flow;interrupting the support of said one surface a fixed distance measuredin the distance of flow while permitting the layer formation flow, dueto its velocity, to continue substantially in the said defineddirection; separating, by blocking part of the layer formation fiow, alayer of a thickness not reater than A of the fixed distance of supportinterruption from the said one surface of the layer formation flow;supporting the one surface of the remainder of the layer formation flowto locally define its direction of travel; successively repeating thesteps of interrupting the support, separating a layer by blocking, andsupporting the remainder between said first receiving position and aspaced first discharge position along a first path generally conformingto a surface generated by moving a straight line parallel to itself sothat a given point on the line moves in a plane perpendicular to themoving line while maintaining a predetermined minimum velocity in saidlayer formation flow along said first path; collecting the particles andsuspending liquid blocked from the layer formation flow during itsmovement along said first path; establishing at a second receivingposition a layer formation fiow of the collected particles andsuspending liquid having a minimum initial velocity; supporting onesurface of the layer formation flow of said collected particles andsuspending liquid to locally define the direction of travel thereof;interrupting the support of said one surface a distance measured in thedirection of flow of at least 1/1.9 of the distance of supportinterruption of said first mentioned layer formation fiow whilepermitting the layer formation flow of said collected particles andsuspending liquid, due to its velocity, to continue substantially in thesaid defined direction; separating, by blocking part of the layerformation flow of the collected particles and suspending liquid, a layerof a thickness approximately A of the fixed distance of supportinterruption from said one surface of the layer formation flow ofcollected particles and suspending liquid; supporting the one surface ofthe remainder of the layer formation how of the collected particles andsuspending liquid to locally define its direction of travel; andsuccessively repeating the steps of interrupting the support, separatinga layer by blocking, and supporting the remainder between said secondreceiving position and a spaced second discharge position along a secondpath generally conforming to a surface generated by moving a straightline parallel to itself so that a given point on the line moves in aplane perpendicular to the moving line While maintaining 'apredetermined minimum velocity in the layer formation fiow of collectedparticles and suspending liquid along said second path.

(References on following page) References Cited in the file of thispatent UNITED STATES PATENTS McDougall July 18, 1905 Inman Aug. 23, 1910Liggett et a1. Apr. 13, 1915 Smith Apr. 20, 1915 Rakowsky June 26, 1945OTHER REFERENCES Publication AV4 of the Second International CoalPreparation Congress at Essen, Germany, September 20 to 25, 1954,dealing with The Dutch State Mines Sieve Bend by F. I. Fontein.

UNITED STATES PATENT OFFICE CERTIFICATE OEF CORRECTION Patent No,2,976,996 R I March 28;, 1 961 I Freerk Fontein It is hereby certifiedthat error appears in the above numbered patent requiring correction andthat the saitiisLetters Patent. should read as corrected below.

Column 1,; 'line' 34 strike out "ame".

Signed and sealed this 21st day of November-1961.,

I (SEAL).

Attest:

Attesting Officer Commissioner of Patents USCOMM-DC

